Liquid crystal displays, plasma displays, organic EL displays, and inorganic EL displays have been known as flat panel displays in which thin-film transistors are used that are driven by an active matrix scheme. In the flat panel display in which the thin-film transistors are used, lines consisting of metal films closely adhere to the surface of a glass substrate in a lattice shape, and the thin-film transistors are provided at the intersection points of the lattice-shaped lines consisting of metal film.
As shown in a schematic cross-sectional view of FIG. 5, the thin-film transistor includes a gate electrode film 2 which consists of a pure copper film formed on the surface of a glass substrate 1, a silicon nitride (SiNx) film 3 formed on the gate electrode film 2 and the glass substrate 1, an n− amorphous Si semiconductor film 4 formed on the silicon nitride (SiNx) film 3, an n+ amorphous Si ohmic film 4′ formed on the n− amorphous Si semiconductor film 4, and a drain electrode film 5 and a source electrode film 6 which consists of pure copper and are formed on the n+ amorphous Si ohmic film 4′.
The thin-film transistor having the layer structure is manufactured as follows. First, as shown in a cross-sectional view of FIG. 6, the gate electrode film 2 consisting of pure copper is formed on the surface of the glass substrate 1, and the silicon nitride (SiNx) film 3 is formed on the gate electrode film 2 and the glass substrate 1. Then, the n− amorphous Si semiconductor film 4 is formed on the silicon nitride (SiNx) film 3, and the n+ amorphous Si ohmic film 4′ is formed on the n− amorphous Si semiconductor film 4. Thereafter, a pure copper film 8 is formed so as to cover the entire surface of the n+ amorphous Si ohmic film 4′. In this way, a laminate 9 is manufactured.
Then, in the laminate 9 shown in FIG. 6, wet etching is performed on a portion of the pure copper film 8 immediately above the gate electrode 2, and plasma etching is performed on the n+ amorphous Si ohmic film 4′. Thereby, an isolation trench 7 is formed, and the n− amorphous Si semiconductor film 4 is exposed. In this way, the drain electrode film 5 and the source electrode film 6 are formed. By the above-mentioned process, a thin-film transistor intermediate (an intermediate of a thin-film transistor) 10 shown in the cross-sectional view of FIG. 5 is manufactured.
Even when plasma etching is performed on only the n+ amorphous Si ohmic film 4′ in the laminate 9 in order to form the isolation trench 7, the surface of the n− amorphous Si semiconductor film 4 cannot be escaped from being affected by the plasma etching since it is exposed to the plasma etching. Therefore, the surface of the n− amorphous Si semiconductor film 4 which is exposed through the isolation trench 7 becomes rough, and an amount of dangling-bonds increases, which causes a surface defect. The surface defect increases an off-current of the thin-film transistor. As a result, the contrast of an LCD is reduced or an angle of view of the LCD is reduced.
In order to solve the above-mentioned problems, a technique has been known in which a hydrogen plasma treatment is performed on the surface of the n− amorphous Si semiconductor film 4 which is exposed through the isolation trench 7, and dangling-bonds on the surface of the n− amorphous Si semiconductor film 4 are combined with hydrogen atoms by the hydrogen plasma treatment so as to stabilize the n− amorphous Si semiconductor film; and thereby, a leakage current is reduced. It has been known that the hydrogen plasma treatment is preferably performed under conditions where a 100% hydrogen gas is used, a hydrogen gas flow rate is in a range of 10 SCCM to 1000 SCCM, a hydrogen gas pressure is in a range of 10 Pa to 500 Pa, an RF current density is in a range of 0.005 W/cm2 to 0.5 W/cm2, and a process time is in a range of 1 minute to 60 minutes (see Patent Document 1).
In the case where Si in the n+ amorphous Si ohmic film 4′ is diffused into the drain electrode film 5 and the source electrode film 6, the specific resistances of the drain electrode film 5 and the source electrode film 6 increase. Although not shown in the drawings, in order to prevent the increasing of the specific resistance of the drain electrode film 5 and the source electrode film 6, a technique has been known in which a barrier film is formed between the n+ amorphous Si ohmic film 4′ and the drain electrode film 5, another barrier film is also formed between the n+ amorphous Si ohmic film 4′ and the source electrode film 6, and a Mo film, a Mo alloy film, a Ti film or a Ti alloy film is generally used as the barrier film (see Patent Document 2).
In general, a pure copper film is used as the drain electrode film 5 and the source electrode film 6. The pure copper film has low adhesion to a ceramic substrate made of glass, alumina, or silicon dioxide. In order to improve the adhesion to the ceramic substrate, a technique has been known in which a copper film including oxygen is formed as an underlayer on the surface of the ceramic substrate, and a pure copper film is formed on the underlayer, which is the copper film including oxygen; and thereby, a composite copper film is obtained (see Patent Document 3). In the composite copper film, the copper film including oxygen comes into contact with the ceramic substrate. In this way, it is possible to improve the adhesion to the ceramic substrate.
As described above, in a process of manufacturing the thin-film transistor, it is required to conduct the hydrogen plasma treatment process for combining dangling-bonds on the surface of the n− amorphous Si semiconductor film 4 with hydrogen atoms so as to stabilize the n− amorphous Si semiconductor film. However, in the case where the hydrogen plasma treatment is performed, the adhesion of the drain electrode film and the source electrode film consisting of pure copper films, to the n+ amorphous Si ohmic film 4′ is reduced.
In order to prevent the reduction of the adhesion, known composite copper films which consisted of a copper film including oxygen as an underlayer, and a pure copper film formed on the underlayer were used as a drain electrode film and a source electrode film. However, the inventors found that, in the composite copper film after the hydrogen plasma treatment, sufficient adhesion to the n+ amorphous Si ohmic film 4′ was not obtained, and the drain electrode film and the source electrode film peeled off, which might cause a defect in the thin-film transistor.